The underlying genetic basis of variation in social behavior is of intense interest, yet only a handful of genes have been linked to specific social behaviors in vertebrates. Thus, there is a strong need to identify populations, human or otherwise, in which there is clear linkage between genes and social behavior. In the proposed project, resources will be developed to take advantage of a uniquely suited model, the white-throated sparrow. Males and females of this abundant North American songbird occur in two plumage morphs that differ with respect to the presence of a chromosomal rearrangement (ZAL2m) that predicts responses to social threat. Birds of the white-striped (WS) plumage morph (ZAL2m/ZAL2) respond to a territorial intrusion with high levels of vocal aggression, whereas birds of the tan-striped (TS) morph (ZAL2/ZAL2) respond with relatively little or no vocal aggression. The morphs also differ with respect to the formation of social attachments and parental provisioning rates; the phenotypes are thus characterized by a suite of correlated complex traits with a discrete genetic basis. The long-term goal of this research program is to fully exploit this unique model organism, which resembles humans with respect to many aspects of social behavior, to link gene expression and complex behavior in ways never before possible. Limited gene flow between the ZAL2 and ZAL2m haplotypes has led to the genetic differentiation of the rearranged chromosomal region, resulting in the accumulation of single nucleotide polymorphisms and other changes. The primary objective of this proposal is to assess the impact of these genetic forces on the genome and brain transcriptome, thus laying the groundwork to identify molecular mechanisms of behavioral dysregulation in future studies. We will combine the experience of two PIs: one with expertise in the behavioral neuroendocrinology of wild sparrows and the other in genome evolution. In Aim 1, we will identify and evaluate sequence differences between the two haplotypes, which will reveal a large number of potential functional polymorphisms that can then be explored experimentally. In Aim 2, we will use Next Generation techniques to sequence total mRNA from individuals for whom reactive aggression was quantified in a natural setting. We will then use weighted gene coexpression network analysis (WGCNA) of the mapped and quantified reads to identify modules of highly correlated genes associated with morph and reactive aggression. Together, the two aims will reveal candidate mechanisms underlying social strategies. This exploratory project will focus on responses to social threat, which are difficult to study in humans in a naturalistic setting. All behavioral manipulations and measurement will be conducted in the animals' natural habitat, making this project highly innovative. The project is significant because many mental disorders-including autism, depression, bipolar disorder and schizophrenia are characterized by dysregulated responses to social threat and because reactive aggression is often comorbid with risk-taking, substance abuse, and criminal behavior. Thus an understanding of the mechanisms underlying response to social threat is important for human health.